ps 
TOPOGRAPHY OF THE BODY. 15 
rest with stability on the water; a duck or a diver shows this well. Speak- 
ing of shape of body, I must allude to the 
§ 34. Centre or Gravity of a bird, and show the admirable provision 
by which this is kept beneath the centre of the body. The enormous breast- 
muscles of a bird are its heaviest parts; sometimes they weigh, to speak 
roundly, as much as one-sixth of the whole bird. Now these are they that 
effect all the movements of the wings at the shoulder-joint, lifting as well as 
lowering the wings; did they all pull straight, the lifters would have to be 
above the shoulder; but they all lie below, and the lifters accomplish their 
office by running through a pulley, which changes their line of traction ; they 
work, in short, like men hoisting sails from the deck of a vessel; and thus, 
like a ship’s cargo, a bird’s chief weight is kept below the centre of motion, 
Topheaviness is further obviated by the fact that birds with a long, heavy 
neck and head draw this in upon the breast, and extend the legs behind, as is 
well shown in a heron flying. The nice adjustment of balance by the yari- 
able extension of the head and legs is exactly like that produced by shifting 
the weight along the bar of a steel-yard ; this, with the slinging of the chief 
weight under the wings instead of over or even between them, enables a 
bird to keep right side up in flight, without exertion. 
Sub-sect.1. Of the Body; its Topography, ete. 
§ 35. Busipes being divided as above into body and members, the exte- 
rior of a bird is further subdivided ; the body being mapped out, mainly for 
purposes of description, into regions, and the members being similarly re- 
solved into their component parts or organs. We have first to notice, as 
the most general, the 
§ 386. Uprer anp Unper Parts. Draw a line from the corner of the 
mouth along the side of the neck to and through the shoulder-joint and 
thence along the side of the body to the root of the tail; all above this line, 
including upper surface of wings and tail, are upper parts; all below, includ- 
ing under surfaces of wings and tail, are under parts; called respectively, 
“above” and “below.” The distinction is purely arbitrary, but so conven- 
ient that it is practically indispensable ; for it will be seen in a moment, how 
an otherwise lengthy description can be compressed into, for example, four 
words: “above, green; below, yellow:” and these terms are often used 
because many birds’ colors have some such simple general character. 
The “upper parts” of the body proper (§ 33) have, also, received the gen- 
eral name of nofeeum (Gr. notos, back; fig. 4, 12) : the “under parts,” simi- 
larly restricted, that of gastreum (Gr. gaster, belly ; fig. 4, 20). These two 
are 
§ 37. Never Nakep, while both head and neck may be variously bare of 
feathers. The only exception is the transient condition of certain birds dur- 
ing incubation: when, either, like the eider duck, they pull feathers off the 
belly to cover the eggs or even to build the nest, or, like several other birds, 
the plumage below is worn off in setting. The gastreum is rarely pecu- 
